from HAVING

The bug is caused by refixing of the constant subquery in pushdown from
HAVING into WHERE optimization.

Similarly to MDEV-29363 in the problematic query two references of the
constant subquery are used. After the pushdown one of the references of the
subquery is pushed into WHERE-clause and the second one remains as the part
of the HAVING-clause.
Before the represented fix, the constant subquery reference that was going to
be pushed into WHERE was cleaned up and fixed. That caused the changes of
the subquery itself and, therefore, changes for the second reference that
remained in HAVING. These changes caused a crash.

To fix this problem all constant objects that are going to be pushed into
WHERE should be marked with an IMMUTABLE_FL flag. Objects marked with this
flag are not cleaned up or fixed in the pushdown optimization.

Approved by Igor Babaev <igor@mariadb.com>